34 - Electromagnetic Waves

34 - Electromagnetic Waves - Chapter 34 Electromagnetic...

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Chapter 34 Electromagnetic Waves CHAPTE R OUTLI N E 34.1 Maxwell’s Equations and Hertz’s Discoveries 34.2 Plane Electromagnetic Waves 34.3 Energy Carried by Electromagnetic Waves 34.4 Momentum and Radiation Pressure 34.5 Production of Electromagnetic Waves by an Antenna 34.6 The Spectrum of Electromagnetic Waves 1066 Electromagnetic waves cover a broad spectrum of wavelengths, with waves in various wavelength ranges having distinct properties. These images of the Crab Nebula show different structure for observations made with waves of various wavelengths. The images (clockwise starting from the upper left) were taken with x-rays, visible light, radio waves, and infrared waves. (upper left—NASA/CXC/SAO; upper right—Palomar Observatory; lower right—VLA/NRAO; lower left—WM Keck Observatory)
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T he waves described in Chapters 16, 17, and 18 are mechanical waves. By definition, the propagation of mechanical disturbances—such as sound waves, water waves, and waves on a string—requires the presence of a medium. This chapter is concerned with the properties of electromagnetic waves, which (unlike mechanical waves) can propa- gate through empty space. In Section 31.7 we gave a brief description of Maxwell’s equations, which form the theoretical basis of all electromagnetic phenomena. The consequences of Maxwell’s equations are far-reaching and dramatic. The Ampère–Maxwell law predicts that a time-varying electric field produces a magnetic field, just as Faraday’s law tells us that a time-varying magnetic field produces an electric field. Astonishingly, Maxwell’s equations also predict the existence of electromagnetic waves that propagate through space at the speed of light c . This chapter begins with a discussion of how Heinrich Hertz confirmed Maxwell’s prediction when he generated and detected electromagnetic waves in 1887. That discovery has led to many practical communication systems, including radio, television, radar, and opto-electronics. On a conceptual level, Maxwell unified the subjects of light and electromagnetism by devel- oping the idea that light is a form of electromagnetic radiation. Next, we learn how electromagnetic waves are generated by oscillating electric charges. The waves consist of oscillating electric and magnetic fields at right angles to each other and to the direction of wave propagation. Thus, electromagnetic waves are transverse waves. The waves radiated from the oscillating charges can be detected at great distances. Furthermore, electromagnetic waves carry energy and momentum and hence can exert pressure on a surface. The chapter concludes with a look at the wide range of frequencies covered by electromagnetic waves. For example, radio waves (frequencies of about 10 7 Hz) are electromagnetic waves produced by oscillating currents in a radio tower’s transmitting antenna. Light waves are a high-frequency form of electromagnetic radiation (about 10 14 Hz) produced by oscillating electrons in atoms.
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